1. Field of the Invention
This invention relates to a technique for offsetting the deflection of an ion beam due to a geomagnetic field or a magnetic field intruding into an ion optics from another device and thus forming the ion beam spot on a sample at substantially the same position as if there is no magnetic field and a technique for avoiding the split of a beam spot which otherwise might occur in the case where the ion beam contains a plurality of types of isotopes.
2. Description of the Related Art
A FIB (focused ion beam) apparatus is used in practical applications to microprocess a sample or to observe an image of the sample by radiating a finely focused ion beam on the sample. The ion beam is deflected by the Lorentz force in the presence of a magnetic field on the optical axis. The accelerating voltage of the FIB apparatus is normally about several tens of kV, and therefore the beam spot may be displace several tens of μm or more by the geomagnetic field. Further, the normally used ion specie Ga Ga contains two types of isotopes Ga69 and Ga71 deflected to different degrees by a magnetic field, thereby posing the problem that the beam is split into two. The ion beam is split also in the case where ions form a cluster. For the beam spot displacement of about several tens of μm, the distance between the two types of ion beams split may reach about 1 μm depending on the difference of the mass-to-charge ratio. It is essential to avoid or suppress this phenomenon for the FIB apparatus which is used for the purpose of microprocessing on the order of nanometer. The simplest method of excluding the magnetic field on the optical axis is to magnetically shield the housing by covering it with a magnetic material as in the prior art. JP-A-11-329318 discloses a technique for magnetically shielding also the forward end portion of the FIB apparatus.
It is difficult, however, to magnetically shield a sample including the neighborhood thereof completely. The sample and the neighborhood thereof could be magnetically shielded almost completely if the whole apparatus is covered with a magnetic shield. It is often desired to form a hole in the magnetic shield, and in such a case, the magnetic field intrudes by way of the hole.
The FIB apparatus normally comprises an electric deflector for radiating the ion beam at the desired position on a sample. Although the displacement of the ion beam spot on the sample due to a magnetic field can be canceled using the electric deflector, it is impossible to prevent the separation of isotopes at the same time. The Wien filter is another means known to deflect the ion beam positively. However, this filter is used rather the purpose of strongly separating isotopes and removing the unnecessary isotope components by bombarding a wall (JP-A-7-296756). Therefore, the exit of the ion beam is very narrow, and in the presence of an external magnetic field, it is difficult for the ion beam to pass through the exit. In view of this, demand has arisen for a technique by which neither the ion beam spot is displaced nor isotopes separated even in the presence of a magnetic field on the optical axis of the ion beam.
This problem is more serious for the FIB-SEM comprising a FIB column and a SEM column combined with each other. The FIB-SEM has recently began to find practical applications as a combination of the observation SEM (scanning electron microscope) and the FIB apparatus to observe a sample processed by the FIB apparatus with a higher resolution. The SEM, which normally has an electromagnet as an objective lens, is required to use the type of a lens called the semi-in or snorkel lens leaking a magnetic field toward the sample to achieve a higher resolution. This magnetic field intrudes into an area on the optical axis of the FIB apparatus and strongly deflects the ion beam. In the case where the ion beam is configured of a plurality of types of beams having different mass-to-charge ratios, therefore, these beams are split from each other. In view of the fact that the ion beam is required to be radiated on a sample in the vicinity of the objective lens of the SEM, on the other hand, the optical axis of the ion beam cannot be magnetically shielded sufficiently. Another problem is that the arrangement of a magnetic shield in the vicinity of the SEM objective lens disturbs the magnetic field of the SEM objective lens and adversely affects the resolution of the SEM.
No technique has been disclosed to solve this problem. Under the circumstances, a FIB-SEM application using the SEM leaking the magnetic field to the neighborhood of a sample uses a method in which the magnetic field of the objective lens of the SEM is suspended during the microprocessing of the sample by the FIB apparatus while the FIB apparatus is stopped during the observation of the sample under the SEM. Even after the exciting current of the objective lens of the SEM is stopped, however, the magnetic field remains. This residual magnetic field changes with time, thereby posing the problem that the ion beam spot also moves with time. To avoid this problem, JP-A-11-329320 discloses a technique in which a demagnetization coil to remove the residual magnetic field is arranged in the neighborhood of the objective lens of the SEM. This method is bothersome to execute, however, in view of the need of demagnetization of the SEM objective lens each time the operation is switched from the SEM to the FIB apparatus.
In microprocessing a sample by the FIB apparatus while at the same time observing the sample under the SEM, an out lens not leaking the magnetic field is conventionally used as an objective lens of the SEM. With the increase in demand for a higher resolution of the SEM, however, the use of an objective lens of semi-in type has become unavoidable. Thus, a technique is in demand to realize the FIB apparatus and the FIB-SEM in which neither isotopes of the ion beams are not separated nor the position of the ion beam spot is not changed on the sample against the existence or a change of a magnetic field on the optical axis of the ion beam.